The present invention relates generally to propagation of light, and more particularly to propagation of light that is to be sensed.
Fuhr, P. L., “Measuring with Light”, Sensors Magazine Online, May 2000, pp. 1-11, available at www.sensorsmag.com/articles/0500/26/, describes sensors that are sometimes referred to as fiber-optic sensors. Fiber-optic sensors have advantages over conventional electrical- and electromechanical-based sensors, stemming mainly from the fact that the fibers are made of nonconducting glass and photons, not electrons, are the signal propagation elements; as a result, the sensors are immune to electromagnetic interference (EMI) and can operate in harsh environmental conditions, offering a geometric versatility that allows unobtrusive sensing. More than 60 different parameters can be measured using fiber-optic sensors. In extrinsic fiber-optic sensors, the optical fiber acts as a transmit/receive light conduit, with signal modulation occurring outside of the fiber, such as in a modulation region that receives light of known parametric values and provides light with a changed characteristic. In intrinsic fiber-optic sensors, on the other hand, an external perturbation directly interacts with the optical fiber and modulates the light signal in the fiber, such as by changing the optical fiber's waveguide controlling boundary conditions.
Various types of optic-fiber sensors as described by Fuhr have been developed. Many fiber-optic sensors are based on Fiber Bragg Gratings (FBGs), which can be fabricated by exposing a photosensitive optical fiber to a periodic pattern of strong ultraviolet light or by etching a periodic pattern directly into the core of the fiber, forming a periodic modulation of the refractive index along the core. Plastic optical fibers (POF) have been applied to sensing in the form of diffracting structures in single- and multi-mode POF with various fabrication techniques. Photonic crystal sensors are the two- and three-dimensional analogs to FBGs, with a periodic modulation of the refractive index in all directions resulting in special reflection and transmission properties. In addition to other applications, various fiber-optic sensors and other optical sensors have been proposed for use in biosensing.
In fiber-optic sensors that indicate stimulus change in the form of wavelength shift in output light, additional systems have been developed for detecting the wavelength shift. Some examples include a broadband light source in combination with a spectrum analyzer and, alternatively, a tunable laser with a narrow line width, sweeping periodically across the reflectivity peak or resonance dip of the sensor cavity.
Othonos, A., and Kalli, K., Fiber Bragg Gratings, Artech House Publishers, Boston, 1999, pp. 304-330, provide an overview of readout techniques for FBGs.
U.S. Pat. No. 6,785,002 describes a variable filter-based optical spectrometer that uses a tapered Fabry-Perot type linear variable optical filter in conjunction with an optical detector array. The input signal is typically carried on an optical fiber and lenses are used to expand and deliver the essentially point source of light from a fiber end into an optical beam that illuminates the variable filter.
U.S. Pat. No. 5,166,755 describes a spectrometer apparatus in which a spectrum resolving sensor contains an opto-electronic monolithic array of photosensitive elements and a continuous variable optical filter. The filter can include a variable thickness coating formed into a wedge shape on a substrate or directly on the surface of the array. If polychromatic light passes through the variable filter and is spectrally resolved before incidence on the array, the output of all the elements in the array provides the spectral contents of the polychromatic light. High spectral resolving power is obtained by subtracting the output signals of adjacent elements in the array. Non-imaging applications include measurement of spectral transmission through samples; for molecular absorption and emission spectra; for spectral reflectance measurements; for pollution and emission control by measuring transmission or absorption; for astronomical spectral analyses of stellar radiation; for pyrometry by measuring thermal radiation; and underwater spectrometry.
It would be advantageous to have improved techniques for structures that propagate light that is to be sensed.